1. Field of the Invention
This invention relates to a zoom optical system for use in an optical apparatus such as an image projecting apparatus or an exposing apparatus.
2. Related Background Art
Some of such optical apparatuses as mentioned above require a zoom optical system excellent in telecentricity on the object side, and excellent in the invariability of the positions of the object surface, the image plane and the exit pupil relative to the variation of the focal length.
For example, Japanese Patent Application Laid-open No. 2002-207167 discloses, as a zoom optical system suitable for an illuminating optical system mounted on a projection exposing apparatus, a zoom optical system in which as the focal length becomes shorter, a lens unit having negative refractive power or a lens unit including a lens element having strong negative refractive power is moved toward the object side, whereby the positions of the object surface, the image plane, the entrance pupil and the exit pupil are made immovable relative to the variation of the focal length.
Also, Japanese Patent Application Laid-open No. 2002-055279 discloses a zoom optical system suitable for a transmitting optical system in an illuminating optical system mounted on a projection exposing apparatus. This zoom optical system has at least four lens units, and at least three lens units are moved, whereby during zooming from the wide angle end to the telephoto end, the refractive power arrangement of positive, negative and positive in succession from the object side is approached from the refractive power arrangement of negative, positive and positive or positive, positive and negative in succession from the object side, whereby the positions of the object surface, the image plane, the entrance pupil and the exit pupil are made immovable relative to the variation of the focal length.
Now, it is also conceivable for the zoom optical system disclosed in Japanese Patent Application Laid-open No. 2002-207167 or Japanese Patent Application Laid-open No. 2002-055279 to be used in the projection optical system of an image projecting apparatus for enlarging and projecting the image of an original formed on a liquid crystal panel or a portion of the projection optical system.
However, when a liquid crystal panel is disposed on the reduction side focal plane of the zoom optical system of Japanese Patent Application Laid-open No. 2002-207167, the back focus cannot be sufficiently secured, and a space for disposing a color synthesizing member therein cannot be secured on the reduction side. In a so-called three-plate type image projecting apparatus, a color synthesizing member for synthesizing three red, green and blue color lights is disposed on the reduction side of the zoom optical system, but if as described above, the back focus cannot be sufficiently secured, it will become impossible to secure the space for disposing the color synthesizing member therein.
Also, in the zoom optical system disclosed in Japanese Patent Application Laid-open No. 2002-207167, the lens unit most adjacent to the reduction side has negative refractive power, and as the focal length becomes shorter, a lens unit having positive refractive power and disposed more adjacent to the magnification side than the negative lens unit is moved toward the magnification side, and the interval between the two lens units is increased. Therefore, the effective diameter of this positive lens unit becomes large. Further, since the lens unit most adjacent to the reduction side has negative refractive power, the numerical aperture (NA) of the reduction side becomes small.
In Japanese Patent Application Laid-open No. 2002-207167, there is also disclosed a case where the lens unit most adjacent to the reduction side is given positive refractive power, but as the focal length becomes shorter, this lens unit is greatly moved toward the magnification side and therefore, if the NA of the reduction side is made great, the effective diameter of this lens unit will become large.
That is, in this zoom optical system, in the state of a minimum focal length (wide), a plurality of lens units deviate toward an aperture stop side, and are moved toward the reduction side focal plane as the focal length becomes longer and therefore, the effective diameters of the lens units are liable to become large, and this is unsuitable for compactness. Further, the lens unit most adjacent to the magnification side has positive refractive power, and this is also unsuitable for securing a sufficient back focus. Moreover, this zoom optical system supposes the use of a light source of a single wavelength, and is not suited for the projection display of a color image.
Also, in the zoom optical system disclosed in Japanese Patent Application Laid-open No. 2002-055279, there is a case where the synthesized refractive power of adjacent lens units becomes negative, and the refractive power is small. Accordingly, the focal length is as great as at least 190 mm. This is because this zoom optical system is an optical system suited for an exposing apparatus, and this zoom optical system is unsuitable for the optical system of a projector or the like of which the downsizing and wider angle are desired. Also, this zoom optical system, like the zoom optical system disclosed in Japanese Patent Application Laid-open No. 2002-207167, supposes the use of a light source of a single wavelength and therefore, is not suited for the projection display of a color image.